In general, the video camera is an apparatus for converting an optical image of an object to a video signal into an electric signal, The video signals in the form of electric signals include a green color (hereinafter, referred to as "G") signal, a red color (hereinafter referred to as "R") signal, and a blue color (hereinafter, referred to as "B") signal, of three primary colors, and the three primary color signals are signals separated by a prism and converted into electric signals by an image pick-up tube. The three primary color signals lose a balance of the white color display level (hereinafter, referred to as "white level") on a screen according to the lighting condition, so that the color is not correctly reproduced. To automatically control the unbalanced white level of the three primary colors, according to the lighting condition of the object to be photographed, is called an auto-white balance (hereinafter, referred to as "AWB") control.
Also, the three primary color signals include dark current which is varied according to the circuit characteristic. The variation of the dark current changes the reference level (hereinafter, referred to as "pedestal level") of the three primary color signals for representing the level of a state which is not picked-up on the screen, and breaks the pedestal level balance of the three primary color signals. It is called an auto-black balance (hereinafter, referred to as "ABB") control that the pedestal level balance broken by the ambient temperature is automatically controlled.
Basically, the AWB control and the ABB control are used in a same way in that the level balance of the three primary colors is controlled by a comparing operation. But, since the ABB control is used for controlling an average value of G signals to be equal to an average value of R and B signals with an optical signal; and the AWB control is used for controlling the average value of the G signals to be equal to an average value of R and B signals with an optical signal. The AWB circuit and the ABB circuits are separately constituted as described above. Each of the AWB circuit and the ABB circuit includes two comparators to usually compare a G signal with a R signal or with a B signal, respectively, or include a comparator to sequentially compare a G signal with a B signal and a R signal, and an A-D converter for analog to digital (hereinafter, referred to as "A-D") converting of a G signal.
Recently, in case of the ABB control, the black set is also carried out in the step in which the pedestal level balance is controlled by the color signals. Accordingly, the compatibility of the AWB circuit and the ABB circuit becomes even more difficult. As a reference, the black set indicates that the pedestal level of the color signals at the normal state is controlled to be equal to the pedestal level of the color signals at the gain increasing time. This creates a problem in that the pedestal levels at the normal time and at the time when the gain of 18 dB is increased, are equal to each other because these levels are closely related. Thus, since the recent ABB circuit controls the pedestal level balance of color signals after the completion of the black set control, the comprising detector for controlling the black set and the comparing detector for controlling the pedestal level balance of the color signals are separately constituted.
In conclusion, the conventional ABB circuit and the conventional AWB circuit can not be used compatibly; and each of these circuits is very complicated to construct.